Power transmission system of hybrid electric vehicle

ABSTRACT

A power transmission system may include: an input shaft receiving engine torque; a first planetary gear set disposed on the input shaft and including a first sun gear selectively connected to a transmission housing, a first ring gear directly connected to the input shaft, and a first planet carrier; a second planetary gear set disposed on the input shaft and including a second sun gear, a second ring gear selectively connected to the first ring gear and selectively connected to the transmission housing, and a second planet carrier directly connected to the first planet carrier and directly connected to an output gear; a first motor/generator directly connected to the first sun gear; and a second motor/generator directly connected to the second sun gear.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority of Korean Patent ApplicationNumber 10-2012-0144869 filed on Dec. 12, 2012, the entire contents ofwhich application are incorporated herein for all purposes by thisreference.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a power transmission system of a hybridelectric vehicle. More particularly, the present invention relates to apower transmission system of a hybrid electric vehicle overcomingdeterioration of system efficiency due to power reuse, reducing capacityof a motor, raising a weight of mechanical power delivery path inhigh-speed driving so as to reduce electric load, and enabling of usingmaximum power of an engine.

2. Description of Related Art

Environmentally-friendly technique of vehicles is very importanttechnique on which survival of future motor industry is dependent.Vehicle makers are focusing on development of environmentally-friendlyvehicles so as to meet environment and fuel consumption regulations.

Some examples of the environmentally-friendly vehicles are an electricvehicle (EV) which is a zero emission vehicle (ZEV) and a fuel cellelectric vehicle (FCEV). The electric vehicle and the fuel cell electricvehicle have merits of no emission, but have limitation due to technicalproblems such as capacitance and life of a battery and socialinfrastructure construction such as charging stations.

Therefore, a hybrid electric vehicle is developed and commercialized.The hybrid electric vehicle generates driving torque by using aconventional internal combustion engine and an electric motor. Thehybrid electric vehicle is a vehicle driven by combining electric powerand power of an internal combustion engine. Because the internalcombustion engine and the electric motor are controlled to operated athigh-efficiency operating points according to the hybrid electricvehicle, efficiency may be excellent and exhaust gas may be reduced.

In addition, construction of charging stations that is problems of theelectric vehicle is unnecessary, fuel economy can be improved, andtravel distances are similar to those of the conventional internalcombustion engine vehicle according to the hybrid electric vehicle.Therefore, it is expected that the hybrid electric vehicles featurefuture environmentally-friendly vehicles.

The hybrid electric vehicle using a power split device such as aplanetary gear set is called a power split type hybrid electric vehicle.Power flow of the power split type hybrid electric vehicle includesmechanical flow where engine torque is directly transmitted to an outputshaft and electric flow where electricity is generated using the enginetorque and a battery is charged by the generated electricity or a motoris driven by energy of the charged battery.

Since the engine can be operated independent from the output shaft inthe power split type hybrid electric vehicle, the engine can be freelypowered on or off during running and an electric vehicle mode can beachieved. In addition, since a power transmission system of the powersplit type hybrid electric vehicle can be operated as an electricallyvariable transmission (EVT) by using two motor/generators, engine may bedriven efficiently.

The information disclosed in this Background section is only forenhancement of understanding of the general background of the inventionand should not be taken as an acknowledgement or any form of suggestionthat this information forms the prior art already known to a personskilled in the art.

SUMMARY OF INVENTION

The present invention has been made in an effort to provide a powertransmission system of a hybrid electric vehicle having advantages ofovercoming deterioration of system efficiency due to power reuse,reducing capacity of a motor, raising a weight of mechanical powerdelivery path in high-speed driving so as to reduce electric load, andenabling of using maximum power of an engine.

A power transmission system of a hybrid electric vehicle according tovarious aspects of the present invention may include: an input shaftreceiving engine torque; a first planetary gear set disposed on theinput shaft and including a first sun gear selectively connected to atransmission housing, a first ring gear directly connected to the inputshaft, and a first planet carrier; a second planetary gear set disposedon the input shaft and including a second sun gear, a second ring gearselectively connected to the first ring gear and selectively connectedto the transmission housing, and a second planet carrier directlyconnected to the first planet carrier and directly connected to anoutput gear; a first motor/generator directly connected to the first sungear; and a second motor/generator directly connected to the second sungear.

The first planetary gear set may be a double pinion planetary gear set,and the second planetary gear set may be a single pinion planetary gearset.

The power transmission system may further include a first clutchdisposed between the first ring gear and the second ring gear, a firstbrake disposed between the second ring gear and the transmissionhousing, and a second brake disposed between the first sun gear and thetransmission housing.

The first brake may be operated at a first electric vehicle mode, thefirst clutch and the first brake may be operated at a second electricvehicle mode, the first brake may be operated at a first hybrid mode,the first clutch may be operated at a second hybrid mode, and the firstclutch and the second brake may be operated at a fixed gear mode.

A power transmission system of a hybrid electric vehicle according tovarious other aspects of the present invention may include: an inputshaft receiving engine torque; a first planetary gear set being a doublepinion planetary gear set and including a first sun gear selectivelyconnected to a transmission housing, a first ring gear directlyconnected to the input shaft, and a first planet carrier; a secondplanetary gear set being a single pinion planetary gear set andincluding a second sun gear, a second ring gear selectively connected tothe first ring gear and selectively connected to the transmissionhousing, and a second planet carrier directly connected to the firstplanet carrier and directly connected to an output gear; a firstmotor/generator directly connected to the first sun gear; a secondmotor/generator directly connected to the second sun gear; a firstclutch disposed between the first ring gear and the second ring gear; afirst brake disposed between the second ring gear and the transmissionhousing; and a second brake disposed between the first sun gear and thetransmission housing.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a power transmission system accordingto the present invention.

FIG. 2 is an operational chart of a power transmission system accordingto the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

Description of components that are not necessary for explaining theillustrated exemplary embodiments will be omitted, and the sameconstituent elements are denoted by the same reference numerals in thisspecification. In the detailed description, ordinal numbers are used fordistinguishing constituent elements having the same terms, and have nospecific meanings.

FIG. 1 is a schematic diagram of a power transmission system accordingto various embodiments of the present invention. Referring to FIG. 1, apower transmission system of a hybrid electric vehicle includes an inputshaft IS, an output gear OG, first and second planetary gear sets PG1and PG2, three friction elements CL1, BK1, and BK2, and twomotor/generators MG1 and MG2.

Therefore, torque transmitted through the input shaft IS and torquetransmitted from the first and second motor/generators MG1 and MG2 areconverted by cooperation of the first and second planetary gear sets PG1and PG2, and then is output through the the output gear OG.

The input shaft IS is an input member, and torque from a crankshaft ofthe engine is transferred or transmitted through a torque converter andis input to the input shaft IS. Instead of the torque converter, anydevice which can transmit the torque of the engine to the powertransmission system may be used. The output gear OG is an output memberand transmits driving torque to a driving wheel through a differentialapparatus.

The first planetary gear set PG1 is a double pinion planetary gear set,and includes a first sun gear S1, a first ring gear R1, and a firstplanet carrier PC1 rotatably supporting a first pinion P1 engaged to thefirst sun gear S1 and the first ring gear R1. The second planetary gearset PG2 is a single pinion planetary gear set, and includes a second sungear S2, a second ring gear R2, and a second planet carrier PC2rotatably supporting a second pinion P2 engaged to the second sun gearS2 and the second ring gear R2. The first planet carrier PC1 is directlyconnected to the second planet carrier PC2, and the first ring gear R1is selectively connected to the second ring gear R2.

The planetary gear sets PG1 and PG2 are combined by the first and secondmotor/generators MG1 and MG2 and friction elements including one clutchCL1 and two brakes BK1 and BK2.

The first and second motor/generators MG1 and MG2 are connected to ahigh voltage battery through an inverter so as to charge the highvoltage battery in regenerative braking and to generate driving torqueby using electricity supplied from the high voltage battery in driving.The first motor/generator MG1 is directly connected to the first sungear S1, and the second motor/generator MG2 is directly connected to thesecond sun gear S2.

In addition, the friction elements are conventional multi-plate frictionelements of wet type that are operated by hydraulic pressure, but arenot limited to these.

The first clutch CL1 is disposed between the first ring gear R1 and thesecond ring gear R2, the first brake BK1 is disposed between the secondring gear R2 and a transmission housing H, and the second brake BK2 isdisposed between the first sun gear S1 and the transmission housing H.Therefore, the first sun gear S1 receives torque from the firstmotor/generator MG1 and are operated as a selective fixed element by thesecond brake B2.

The first and second planet carriers PC1 and PC2 are directly connectedto the output gear OG and are operated as a final output element. Thefirst ring gear R1 is directly connected to the input shaft IS and isoperated as an input element which receives the torque of the engine.The second sun gear S2 receives torque from the second motor/generatorMG2. The second ring gear R2 is selectively connected to the first ringgear R1 through the first clutch CL1 so as to be operated as a selectiveinput element, and is operated as a selectively fixed element by thefirst brake B1.

FIG. 2 is an operational chart of a power transmission system accordingto various embodiments of the present invention. Referring to FIG. 2,operation of the friction elements at each mode will be described indetail.

The first brake BK1 is operated at a first electric vehicle mode EV1.The first clutch CL1 and the first brake B1 are operated at a secondelectric vehicle mode EV2. The first brake BK1 is operated at a firsthybrid mode EVT1. The first clutch CL1 is operated at a second hybridmode EVT2. The first clutch CL1 and the second brake BK2 are operated ata fixed gear mode FG.

The power transmission system according to various embodiments of thepresent invention can achieve two EV modes, two hybrid modes, and onefixed gear mode. Operation of the power transmission system at each modewill be described in detail.

First electric vehicle mode. If the first brake BK1 is operated andelectricity is applied to the second motor/generator MG2, the torque ofthe second motor/generator MG2 is output through the second planetcarrier PC2 connected to the output gear OG. Therefore, the firstelectric vehicle mode is achieved. Since the engine is stopped at thistime, the first motor/generator MG1 connected to the first sun gear S1rotates to a reverse direction, but it does not have any effect ondriving of the vehicle.

Second electric vehicle mode. If the first clutch CL1 and the firstbrake BK1 are operated, the first and second motor/generators MG1 andMG2 can be used as motors. Therefore, the second electric vehicle modeis achieved. Since the driving torques of two motors are output at thesecond electric vehicle mode, driving torque of the vehicle can bemaximized. Therefore, the first and second motor/generators MG1 and MG2can be used suitably for torque performance needed in the powertransmission system. In addition, capacities of the first and secondmotor/generators MG1 and MG2 can be reduced if the exemplary embodimentsof the present invention are applied to a plug-in hybrid electricvehicle.

First hybrid mode. If the engine is started by the first motor/generatorMG1 in a state that the first brake BK1 is operated, the first hybridmode is achieved. The first motor/generator MG1 is operated as agenerator and controls shift ratio. In addition, since the second ringgear R2 is operated as the fixed element, the second motor/generator MG2cannot control the shift ratio and is operated as the motor so as totransmit the torque to the output gear OG.

Second hybrid mode. If the first brake BK1 is released and the firstclutch CL1 is operated at the first hybrid mode, the first ring gear R1and the second ring gear R2 are connected and the second motor/generatorMG2 rotates to the reverse direction. At this state, the firstmotor/generator MG1 is operated as the generator and the secondmotor/generator MG2 is operated as the motor. Therefore, the secondhybrid mode is achieved.

Fixed gear mode FG. If the first clutch CL1 and the second brake BK2 areoperated, the first sun gear S1 is stopped by the second brake BK2, thesecond planet carrier PC2 connected to the output gear OG rotates fasterthan the first ring gear R1 connected to the engine does. Since thefirst motor/generator MG1 does not transmit the torque at this time, theshift ratio is formed only by the second planetary gear set PG2. Inaddition, since the engine is directly connected the first and secondplanetary gear sets PG1 and PG2 at the fixed gear mode, efficiency atthe fixed gear mode is superior to that at power split mode. Inaddition, a parallel hybrid system where the motor as well as the engineis operated can be achieved at the fixed gear mode.

According to the present invention, two electric vehicle modes, twohybrid modes, and one fixed gear mode can be achieved by combining twoplanetary gear sets PG1 and PG2, three friction elements CL1, BK1, andBK2, and two motor/generators MG1 and MG2. In addition, deterioration ofsystem efficiency due to power reuse may be overcome, capacity of amotor may be reduced, a weight of mechanical power delivery path inhigh-speed driving may be raised so as to reduce electric load, andmaximum power of an engine can be used.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. The exemplary embodiments were chosen and described in orderto explain certain principles of the invention and their practicalapplication, to thereby enable others skilled in the art to make andutilize various exemplary embodiments of the present invention, as wellas various alternatives and modifications thereof It is intended thatthe scope of the invention be defined by the Claims appended hereto andtheir equivalents.

What is claimed is:
 1. A power transmission system of a hybrid electricvehicle, comprising: an input shaft receiving engine torque; a firstplanetary gear set disposed on the input shaft and including a first sungear selectively connected to a transmission housing, a first ring geardirectly connected to the input shaft, and a first planet carrier; asecond planetary gear set disposed on the input shaft and including asecond sun gear, a second ring gear selectively connected to the firstring gear and selectively connected to the transmission housing, and asecond planet carrier directly connected to the first planet carrier anddirectly connected to an output gear; a first motor/generator directlyconnected to the first sun gear; and a second motor/generator directlyconnected to the second sun gear.
 2. The power transmission system ofclaim 1, wherein the first planetary gear set is a double pinionplanetary gear set, and the second planetary gear set is a single pinionplanetary gear set.
 3. The power transmission system of claim 1, furthercomprising: a first clutch disposed between the first ring gear and thesecond ring gear; a first brake disposed between the second ring gearand the transmission housing; and a second brake disposed between thefirst sun gear and the transmission housing.
 4. The power transmissionsystem of claim 3, wherein the first brake is operated at a firstelectric vehicle mode, the first clutch and the first brake are operatedat a second electric vehicle mode, the first brake is operated at afirst hybrid mode, the first clutch is operated at a second hybrid mode,and the first clutch and the second brake are operated at a fixed gearmode.
 5. A power transmission system of a hybrid electric vehicle,comprising: an input shaft receiving engine torque; a first planetarygear set being a double pinion planetary gear set and including a firstsun gear selectively connected to a transmission housing, a first ringgear directly connected to the input shaft, and a first planet carrier;a second planetary gear set being a single pinion planetary gear set andincluding a second sun gear, a second ring gear selectively connected tothe first ring gear and selectively connected to the transmissionhousing, and a second planet carrier directly connected to the firstplanet carrier and directly connected to an output gear; a firstmotor/generator directly connected to the first sun gear; a secondmotor/generator directly connected to the second sun gear; a firstclutch disposed between the first ring gear and the second ring gear; afirst brake disposed between the second ring gear and the transmissionhousing; and a second brake disposed between the first sun gear and thetransmission housing.